The present invention relates generally to excitable tissue stimulation, and more particularly to improvements in energy efficiency of implantable neurostimulators utilized in conjunction with one or more electrodes which are surgically implanted on a selected nerve or nerves of the patient to permit electrical stimulation thereof.
Implantable neurostimulators employ one or more batteries as the power source for the device When the power is depleted, it becomes necessary to perform surgical removal and replacement of the device. It is desirable, therefore, to conserve device energy to the extent possible, while supplying stimulation as needed, in order to prolong device lifetime and increase the interval between surgical replacements. Of course, such considerations are important for any implantable medical device, not merely neurostimulating devices Ideally, cardiac and neuromuscular stimulators have output circuits which may characterize them as constant voltage or constant current devices. In practice, however, presently available implantable versions of these devices depart significantly from the constant output ideal models.
The constant current mode of the existing devices has the advantage that changes in system resistance or impedance do not affect the output current of the device. For neurostimulation, this is important because it is the current applied to the nerve which causes excitable fiber depolarization. The constant current mode also tends to limit the possibility of damage to the nerve being stimulated. Nerve damage might occur, for example, from any overstimulation resulting from changes in system impedance that cause current variations.
Despite its advantages, however, constant current stimulation has certain drawbacks A constant output current system generally requires that the supply voltage must be significantly greater than the voltage which is actually delivered to the nerve electrode. This excess voltage is dropped across the constant current regulating circuit and may represent significant energy waste, which, for reasons noted above, results in the need for undesirably frequent replacements of implanted systems. Another problem is the excess energy consumption in the control circuit of the constant current system. This circuit is required to have a sufficiently rapid response time to permit controlling the output current during even very brief output pulses. Therefore, amplifier bias currents must be set relatively high, with consequent energy overhead The control circuit may be designed without an active amplifier, but would then possess the disadvantage of requiring even greater supply voltage overhead.